This invention relates to crystallization. Crystalline solids have many desirable properties such as purity, size, shape, color, and processability which are commercially advantageous. In addition for many materials crystallization is the best and cheapest method for producing that material industrially.
Crystallization, a process whereby a solid separates from a solution, is well known. Conditions are imposed on the solution which allow a solid phase of crystalline particles to grow from atoms to a size sufficiently large to allow separation by physical methods. The basic principle behind crystallization is that a solution containing one or more solutes is altered by either cooling, removal of solvent, or addition of another substance so that the ability of the solvent to dissolve the solute is reduced, and a fraction of the solute forms a solid phase which can be removed from the mixture. In other words, if a solution in equilibrium between its solid and liquid phases is altered in such a way that the amount of dissolved solids exceeds the equilibrium concentration, the system will seek to reestablish an equilibrium condition by getting rid of the excess solids. The resulting process is crystallization, and the concentration excess, termed supersaturation, is the driving force.
The quantity of solid matter which a liquid can dissolve and the effect on this quantity of changes in temperature, pressure, particle size, and the presence of foreign substances are not predictable. Gravitational phenomena, including convection, sedimentation, and interactions of materials with their containers all affect the crystal growth process. If they are not taken into consideration they can have adverse effects on the quality and on crystal production because both the material transport and the crystal growth process are dominated by gravity-driven convection. The crystallization process, is also strongly affected by the presence and behavior of buoyancy-driven convection. Unless precautions are taken to suppress convection currents, undesirable results are possible.
As a practical matter, convection and sedimentation can be completely eliminated most directly under conditions of low gravity attained during orbital flight. For this reason it is believed that higher quality forms of crystals and drugs can be manufactured in orbit, that is, under microgravity conditions. Consequently there is much current interest and experimentation in crystallization processes conducted without gravitational effects. Such crystallization processes are deemed to lead to better control of the quality and properties of crystals. Electronic measurements on these crystals characterize the crystalline perfection, and thus help to establish the influence of microgravity on crystal growth. Microgravity is about one/millionth the level of gravity on earth. The term "zero gravity" is often used where microgravity is the correct term.
There is one disadvantage to crystallization in space. That is the nucleation process. Because of the absence of convection in a microgravity environment, cooling proceeds by thermal diffusion from the cold walls to the center of the solution chamber. Nucleation, which will first begin where the temperature is the lowest, will occur on and near the chamber walls. Crystals, which nucleate heterogeneously on the walls, will have their growth modified and interfered with by their attachment to the wall. There is, then, a need for a new and improved nucleation process in space. By this invention such a nucleation process is provided.